Microelectronic devices such as semiconductor chips require large numbers of electrical connections in a small area. For example, a complex semiconductor chip may have hundreds of contacts for signal input and output and for power supply, all within an area of a few cm.sup.2. These contacts must be reliably connected to corresponding contact pads on a substrate such as a chip mount, circuit panel or multichip module. The connection between the chip and the substrate must meet numerous, often contradictory requirements and desires. It should be low in cost, and yet highly reliable. It should require only the minimum amount of area on the substrate. Ideally, the connected chip should be accommodated in an area of the substrate approximately equal to that of the chip itself. The connections should be robust and capable of withstanding repeated changes in the temperature of the chip and the substrate, and the associated differential thermal expansion and contraction of the chip and substrate. Moreover, the connection systems should not impose unusual or costly requirements in fabrication of the chip or substrate. The connection system desirably should facilitate testing of the chip, and the connection system itself, before the chip is finally assembled to the substrate. All of these considerations, taken together, represent a formidable engineering challenge. These considerations are present, to a greater or lesser degree, in other connections used in microelectronic devices as, for example, connections between substrates or circuit panels.
Numerous solutions to these problems have been proposed heretofore. One common method of making connections to a semiconductor chip or a similar device is wire bonding. In wire bonding, numerous fine wires are connected to the contact pads of the chip and extend outwardly, over the edges of the chip, to corresponding contact pads on a substrate. The process requires individual connections of each contact on the chip and also requires an area of the substrate substantially larger than the chip itself. In a further method referred to as tape automated bonding or "TAB", fine metallic leads are disposed on a polymeric tape. The leads may be connected individually or en masse to the contact pads of the chip and to the contact pads of the substrate. This method also requires a substantial area of the chip substrate surface. In so-called "flip-chip" bonding, the front or contact-bearing surface of the chip faces downwardly to the substrate, and each contact on the chip is connected to the corresponding contact pad on the substrate by a solder column. This method can mount a chip in an area of the substrate substantially equal to the area of the chip itself, but suffers from drawbacks such as poor resistance to thermal cycling, cost and process difficulties.
Numerous other chip mounting methods have been proposed. For example, Patraw, U.S. Pat. Nos. 4,695,870 and 4,716,049 disclose a package for a microelectronic chip utilizing hollow metallic "compressive pedestals" engaged between the chip and the substrate and making a spring-loaded electrical connection. McMahon, U.S. Pat. No. 5,321,583, discloses a second level interconnect, i.e., an interconnect arranged to connect an already packaged device to a circuit panel. This interconnect uses apparently solid metallic spheres disposed between the package and the circuit panel and an external spring to maintain engagement of all of the components. Tsukagoshi et al., U.S. Pat. No. 5,120,665, discloses an isotropic electrically conductive adhesive with numerous deformable electroconductive particles disposed therein. The particles may include a polymeric core with a thin metallic layer, typically less than about 1 micron thick. These particles are distributed at random between a chip and a substrate, so that those particles which are engaged between contacts of the chip and contact pads on the substrate will permit electrical conduction between the so-engaged pad and contact. As noted in International Interconnection Intelligence Flip Chip Technology Impact Report, pp. 3-79 to 3-80, a Seiko Epson Corporation process uses gold plated resin balls mixed with adhesive and printed onto a substrate contact. The substrate is then engaged with a chip and secured by a separate adhesive. Despite these efforts in the art, still further improvement would be desirable.